Many types of present-day circuits utilize differentially connected field effect transistors (FETs). More particularly, operational amplifiers now include differential field effect transistor input stages and bipolar output stages. The FET input stages provide advantages with respect to differential bipolar transistor input stages such as higher input impedance. Unfortunately, the electrical parameters of a differential pair of field effect transistors varies with temperature. Also the electrical parameters of differential pairs of field effect transistors varies from circuit to circuit because of processing variations.
More particularly, the unity gain bandwidth of such field effect differential pairs is a function of the transconductance thereof which varies with the pinch-off voltage (V.sub.p), the drain-to-source current (I.sub.DS), and the saturation current (I.sub.DSS) thereof. Since the pinch-off voltage and the saturation current are functions of temperature, prior art circuits including differential field effect pairs have utilized bipolar current sources which generate drain-to-source currents having a predetermined temperature co-efficient which compensates somewhat for the temperature coefficients of the pinch-off voltages and the saturation currents of the differential FETs. One such prior art bipolar current source circuit includes a zener diode, a plurality of diodes and a PNP transistor, for instance. Accordingly, this bipolar current source or supply circuit takes up an undesirable amount of the chip area of an integrated circuit thereby increasing costs and reducing reliability.
The saturation current and the pinch-off voltage of the differential field effect transistors are also a function of process variations. Since such field effect transistors are manufactured by a process such as ion implantation which is generally independent of the bipolar process utilized to fabricate bipolar diodes and bipolar transistors, the bipolar current supply does not develop a current which compensates for the changes in these electrical parameters of the field effect transistors over processing. As a result, the transconductance and thus the bandwidth of such field effect transistor differential amplifiers having bipolar current supplies varies undesirably with process variations which results in substantially reduced yield and increased costs. More specifically, if the actual bandwidth of some high frequency, integrated circuit amplifiers having a desired bandwidth of 5 MHz is below 4 MHz, the amplifiers are unsuitable for high frequency amplification. Also, if the actual bandwidth is above 6 MHz, such amplifiers tend to oscillate because of internal phase shifts.
A field effect transistor having the gate thereof connected to the source thereof has been suggested for providing a drive current which varies in such a manner as to compensate for the changes in pinch-off voltage and saturation current over the processing of the differential FET pair. However, this current supply provides a drain-to-source current having a temperature coefficient which aggravates rather than compensates for the change in pinch-off voltage and saturation current over temperature of the differential FET pair. Consequently, this field effect transistor current supply is unsuitable for use with differential FET pairs because the temperature specifications of such circuits is critical.
Thus, the semiconductor industry generally has chosen to utilize the complex bipolar current supplies which decrease yields rather than FET current supplies which fail to meet temperature specifications.
A degenerated or negative feedback, field effect transistor current supply is disclosed by U.S. Pat. No. 4,053,915, entitled, "Temperature Compensated Constant Current Source Device," (Ser. No. 669,065) which was invented by one of the inventors hereof and assigned to the assignee hereof. This current supply utilizes negative feedback and can be designed to provide a current of substantially zero temperature coefficient for use with bipolar transistor differential amplifiers which require such a current. This supply has heretofore been believed to be unsuitable for use with FET differential pairs which, as previously mentioned, have been believed to require a bipolar current supply for providing a current having a temperature coefficient which compensates for the change of saturation current and pinch-off voltage of the differential FETs with temperature. Such negative feedback current source, although suitable for many applications involving bipolar transistors, has the further disadvantage of requiring a source resistor, which sometimes takes up an undesirable amount of chip surface area, and the configuration of the negative feedback current supply provides only limited flexibility in the choice of the geometry of the FET used therein.